This is nothing without a form of security (4)
I believe the answer is:
nail
'form of security' is the definition.
'this is nothing without a' is the wordplay.
'this is nothing' becomes 'nil' (I've seen this before).
'without' means one lot of letters goes inside another ('without' can be similar in meaning to 'outside').
'nil' placed around 'a' is 'NAIL'.
(Other definitions for nail that I've seen before include "metal fixture" , "Metal spike with head" , "Hammer's target" , "it needs hammering home" , "Growth on digit" .)